In recent years, a video display device is becoming widespread that uses an LED (light emitting diode) backlight for illuminating a display panel. The LED backlight has an advantage of enabling the use of a local dimming technique. The local dimming is a technique that divides the backlight into plural regions to control a light emission of the LED for each of the regions depending on a luminance value of a video region corresponding to each of the regions.
When viewing obliquely a video displayed using the local dimming technique, some videos may suffer halos. For example, when viewing obliquely a video including a high-luminance pattern in a pattern with a substantially uniform luminance, there may appear halos due to a light leak around the high-luminance pattern.
FIG. 15 depicts an example of a video with halos appearing thereon. In FIG. 15(A), a video is depicted which includes a white pattern 2 in a substantially uniform gray pattern 1. FIG. 15(A) depicts backlight divided regions 3 superimposed on the video.
FIG. 15(B) depicts light emission luminances 4 and 5 of LEDs along line A-A′ of FIG. 15(A), a backlight luminance distribution 6 obtained by light emission of the LEDs, and an output gradation value 7 of a liquid crystal panel. In the local dimming, the light emission luminances 4 and 5 of the LEDs in each divided region 3 are decided depending on the gradation value of each video region corresponding to each divided region 3. In FIG. 15(B), a maximum value of gradation values of pixels contained in each video region is used as the gradation value of each video region.
In this example, the light emission luminance 4 of the LEDs in the divided regions 3 completely included in the gray pattern 1 is decided to be lower than the light emission luminance 5 of the LEDs in the divided regions 3 completely or partially included in the white pattern 1. The output gradation value 7 of the liquid crystal panel is decided such that the picture quality of a finally displayed video is equivalent to that of an original video, based on the backlight luminance distribution 6 obtained as a result of the LEDs' light emission.
However, in case that there is a large difference between the light emission luminance 4 of the LEDs corresponding to the gray pattern 1 and the light emission luminance 5 of the LEDs corresponding to the white pattern 2, when viewing the video obliquely, a halo may appear around the white pattern 2 due to a light leak as depicted in FIG. 15(C). To restrain such appearance of the halo, there exists a technique for increasing the luminance of LEDs illuminating a dark portion of a video (see Patent Document 1).
FIG. 16 is an explanatory view of this conventional technique. FIG. 16(A) is the same diagram as FIG. 15(A). In this conventional technique, as depicted in FIG. 16(B), the light emission luminance 4 of the LEDs corresponding to the gray pattern 1 is increased so that the difference becomes smaller between the light emission luminance 4 and the light emission luminance 5 of the LEDs corresponding to the white pattern 2. This restrains the halo from appearing around the white pattern 2 as depicted in FIG. 16(C).